1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
Typical image forming apparatuses form an image on paper by performing primarily transfer of a toner image to a surface of an endless transfer belt, which is rotatably stretched over a plurality of rollers including a drive roller, a driven roller, and a backup roller, from a photoreceptor drum (an image carrier), and then performing secondary transfer of the toner image to the paper.
Such image forming apparatuses have a secondary transfer roller that presses the transfer belt against the backup roller. When a portion of the transfer belt on which a toner image has been primarily transferred passes through the backup roller, paper is allowed to pass between the transfer belt and the secondary transfer roller, so that the toner image is secondarily transferred on the paper.
In such image forming apparatuses, electrical discharge may occur across the gap between paper and the transfer belt in the vicinity of the entrance of a transfer nip area where the transfer belt is in proximity to the secondary transfer roller, during the secondary transfer of a toner image on the transfer belt to the paper. This causes disorder of the toner image, leading to formation of an abnormal image called toner scattering.
In the case of image formation on heavy and stiff thick paper, the trailing end of the thick paper is bent by being pre-nipped between the transfer belt and the secondary transfer roller, and the trailing end may hit the transfer belt as a reaction of its restoration. The transfer belt inwardly deflects due to the shock of such hitting, resulting in formation of a gap between the thick paper and the transfer belt. Electrical discharge may occur across the gap, leading to formation of the above-described abnormal image.
In another image forming apparatus, a position of the transfer belt is changed immediately before the thick paper passes between the transfer belt and the secondary transfer roller to reduce the shock due to hitting of the thick paper against the transfer belt (see, for example, Japanese Unexamined Patent Application Publication No. 2010-139603 (JP-A-2010-139603)).
In still another image forming apparatus, an electrode is disposed inside the transfer belt to reduce electrical discharge that may occur on the transfer belt (see, for example, Japanese Unexamined Patent Application Publication No. 2010-2838 (JP-A-2010-2838).
Unfortunately, in the image forming apparatus disclosed in JP-A-2010-139603, the thick paper still hits the transfer belt even though the shock is reduced, and thus the transfer belt still deflects inwardly and a gap is formed, resulting in occurrence of electrical discharge across the gap in some cases. In addition, if the position of the transfer belt varies, the area of paper pre-nipped between the transfer belt and the secondary transfer roller is different between before and after the variation, which may cause color or density change in the formed image.
In the image forming apparatus disclosed in JP-A-2010-2838, paper which is to be nipped for a transfer is conveyed in a direction substantially perpendicular to a line that connects the axis of the secondary transfer roller with the axis of a counter roller. Accordingly, the paper is not pre-nipped, and thus, the paper is not bent. This means that there is no possibility that the paper hits against the transfer belt with its rebound. In addition, the electrode disposed inside the transfer belt does not restrict the inward deflection of the transfer belt.